AN EXTENDED HEXAGON MODEL FOR CU UNDERPOTENTIAL DEPOSITION ON AU(111)

The hard hexagon adsorption isotherm of Baxter and Joyce was previousl y used in a statistical model for the underpotential deposition (upd) of Cu on Au(111). This model gave a plausible physical picture of Cu u pd on Au(111) and was able to reproduce the gross features of the volt ammogram. In the present work the hexagon model is extended to include a mean field treatment of the adsorbate interactions. Using this exte nded hexagon model, coupled adsorption isotherms for copper and sulfat e ions are obtained and solved numerically. These isotherms yield theo retical quartz microbalance curves which compare well with the experim ental ones. The model voltammogram reproduces the experimental voltamm ogram including the finer details of the broad foot region.